Cavity resonator having alternate apertured drift tubes connected to opposite end walls



June 25, 1968 RYOKA SAWADA ET AL 3,390,301

RESONATOR HAVING ALTERNATE APERTURED DRIFT CAVITY TUBES CONNECTED TOOPPOSITE END WALL 2 Sheets-Sheet 1 Filed Dec.

F/G. M PRIOR ART INVENTORS rQYd/ffl Snwnon ATTORNEY June 25,- 1968 RYOKASAWADA ET AL 3,390,301

CAVITY RESONATOR HAVING ALTERNATE APERTURED DRIFT TUBES CONNECTED TOOPPOSITE END WALLS Filed Dec. 9, 1965 2 Sheets-Sheet 2 FIG 4 Y INVENTORSRYO/(A 3/; W000 Yon: m KIM/5K0 ORNEY United States Patent 3,390,301CAVITY RESONATOR HAVING ALTERNATE APERTURED DRIFT TUBES CONNECTED TOOPPOSZTE END WALLS Ryoka Sawada, Kokuhunji-shi, and Yoichi Kaneko,Hachioji-shi, Japan, assignors to Hitachi, Ltd., Tokyo, Japan, acorporation of Japan Filed Dec. 9, 1965, Ser. No. 512,673 Claimspriority, application Japan, Dec. 18, 1964, 39/ 71,061 2 Claims. (Cl.SIS-5.39)

ABSTRACT OF THE DESCLOSURE A cavity resonator in the form of a hollowconductor casing having end walls including aligned apertures positionedon the axis of the casing and along which a beam of charged particles ispropagated, a plurality of aligned apertured members arranged along thepath of the particles Within the casing defining a plurality of gapstherebetween, and supporting conductors interconnecting alternate onesof said apertured members and connected to respective opposite end wallsof the casing.

This invention relates to cavity resonators.

Cavity resonators previously used in klystron amplifiers for microwavefrequency bands and particularly for relatively low frequencies havegenerally been very large in size, having a considerably large outsidediameter even in cases of klystrons of relatively low power level. Thisnaturally has resulted in increase in size of the focusing coil toeconomical disadvantage and, even with high power tubes, it has beenextremely difficult to minimize their external dimensions withoutdetracting from their efficiency. Mere reduction in size of cavityresonators may possibly be realized by the method of increasing theelectrostatic capacitance of the gaps, where the electron beam and theelectric field of the cavity resonator interact, while reducing theirinductance. Cavity resonators so designed, however, would be impracticalbecause of reduction in characteristic impedance and increase in powerloss.

Besides single-cavity resonators referred to above, multicavityresonators have also been in use which correspond to a combination of amultitude of single-cavity resonators and have advantages such asincrease in frequency bandwidth and reduction in power loss. Themulticavity type of resonator, however, provides modes of operationcorresponding in number to the cavities included in the resonator andthis requires separation of any unwanted modes other than the principalone and thus causes a difficult design problem.

Description Will next be made of the structure of a conventional form ofcavity resonator illustrated in FIG- URES laand 1b.

This form of cavity resonator includes two posts 2 formed with alignedaxial holes 3 for passage therethrough of an electron beam, and carriedon opposite conductor walls, which are integral with a hollow conductorcasing 1 enclosing the posts. With this device, reduction in size of thecavity results in proportional reduction in inductance L and incapacitance C of the resonator. In this case, however, since theinductance L of the resonator apparently is determined by its size, thecapacitance C must be increased for the resonator to maintain the samebasic resonant frequency. To meet this requirement, it would benecessary to increasethe cross-sectional area of the posts 2 or toreduce the gap 4 therebetween with the result that the electric field isunnecessarily collected in areas other than those where it is intendedto interact "ice with an electron beam, disadvantageously increasing thepower loss of the resonator. In the case of a multicavity resonator,which is actually a combination of a multitude of single-cavityresonators, it is possible to divide the exciting RF voltage into partscorresponding in number to the resonant cavities and to apply suchpartial voltages to the respective cavities. By doing this, the powerloss of such multicavity ressonator can be reduced while increasing itsfrequency bandwidth. On the other hand, the multicavity resonatorinherently involves a disadvantage in that it provides unwanted modes ofoperation besides the basic resonant frequency.

The present invention has for its primary object to provide a cavityresonator reduced in outside diameter.

Another object of the present invention is to provide a cavity resonatorwhich provides no unwanted modes of operation in the vicinity of itsbasic resonant frequency.

A further object of the present invention is to provide a cavityresonator which is relatively limited in loss and gives a high gain whenemployed, for example, in velocity modulation tubes.

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionwhen taken in conjunction with the accompanying drawings in which:

FIGURES la and 1b provide longitudinal and transverse sectional views,respectively, of a known cavity resonator;

FIGURES 2a and 2b illustrate one form of multiplegap cavity resonatorembodying the present invention;

FIGURES 3a and 3b illustrate another form of multiple-gap cavityresonator embodying the invention;

FIGURE 4 illustrates a further form of cavity resonator embodying thepresent invention;

FIGURES 5a, 5b, and 5c illustrate various forms of equivalent circuitsof the multiple-gap cavity resonator embodying the present invention;

FIGURES 6 and 7 represent diagrams illustrating the operation of theinventive cavity resonator; and

FIGURE 8 illustrates a velocity modulation tube for amplification useincorporating a number of cavity resonators embodying the presentinvention.

In 2a and 2b parts indicated by reference numerals 1, 3 and 4 correspondto those of the conventional resonator shown in la and 1b which bear thesame numerals. Reference numeral 5 indicates apertured members formedeach with an axial aperture for passage therethrough of an electronbeam. These apertured members 5 constitute a modification formultiple-gaps formation of the posts 2 used in the conventionalsingle-gap cavity resonator shown in FIGURES la and 1b and arealternately interconnected by a pair of supporting conductors 6, with aworking gap 4 formed between each two adjacent opposing ones of theape'rtured members. Though the embodiment shown in FIG. 2a has threegaps, it is to be understood that cavity resonators including a greaternumber of working gaps can also be readily realized.

The equivalent circuit of the cavity resonator shown in FIG. 2a isillustrated in FIG. 5a. Reference characters C C and C indicate theelectro-static capacitance of the respective gaps of the resonator,including any additional electro-static capacitance appearing in regionsadjacent to the respective gaps. Characters L and L represent theinductances of the respective supporting conductors 6; L represents theinductance of the cavity; and r a series resistance corresponding to thecavity loss. In cases where the inductances L and L of the supportingconductors are relatively limited, the equivalent circuit isapproximately illustrated as in FIGS. 5b and 50. Assuming that the threegaps have the same capacitance, the entire capacitance apparently isthree times as large as the one for any single gap and, therefore, thecavity inductance L, can be reduced to one-third of that of anequivalent singlegap resonator. This means that this form of cavityresonator can be greatly reduced in cavity size. In addition, with theresonator, constructed as described above, any unwanted modes other thanthe basic resonant frequency can be avoided owing to the single-cavitystructure of the resonator.

Reference will next be made to FIG. 6, which illustrates the electricfield distribution in the working gaps 4 of the inventive cavityresonator at an instant during operation, in the case where theresonator has three gaps, as illustrated in FIG. 2a. As observed, thedistribution of the electric field E is alternately reversed in phasefor the successive gaps and the high-frequency exciting fields takingplace in the respective gaps have substantially the same amplitude sincethe apertured members interconnected by either of the supportingconductors are held at substantially the same potential. In one experiment, however, the magnitude of the voltage appearing in theintermediate gap has been approximately twice as large as thoseappearing in the remaining gaps.

Next, the operation of the cavity resonator when used for velocitymodulation of an electron beam will be described, assuming that theresonator includes an odd number 11 of gaps identical in voltageamplitude and alternately reversed in phase of the electric fielddistributed therein and that the electron transit angle from gap to gapis identical throughout the resonator.

Assuming further that electrons entering the resonator at an initialvelocity of V has an exit velocity of V after passage through themultiple-gap cavity of the resonator, the following relation isgenerally obtained for minute signals:

where V represents the beam voltage, Ve represents the highfrequencyexciting voltage for each gap, and [i represents the beam couplingcoefficient. [3 can also be expressed as follows:

i Cos 2 (3) It follows, therefore, that a coupling coefiicient 11 timesas large as that for a single-gap resonator can be obtained when theelectron transit angle is selected, for example, at 1r radians. Thus, inthe event such multiple-gap cavity is employed for beam modulation, theoverall gap capacity C0 obtained will be 11 times that for a single-gapcavity and the cavity inductance L one-11th of that for the latter. Incases where a series resistance r is selected proportional to L0 or inthe same order as that for a single-gap cavity, the parallel resonanceresistance R (=L0/C01-) will be reduced to between 1/11 and l/11 of thatobtainable with a single-gap cavity and the quality factor Q(=W0L0/1-)of the cavity to 1/11. In the case of a multiple-gap cavity, however,because of its high beam coupling coefiicient, the high-frequencyexciting voltage for each gap can be reduced to one-11th, providing fora velocity modulation equivalent to that of a single-gap cavity, and thepower loss (=V /2Rs/1) of the cavity, at most equal to that of asingle-gap cavity,

can be reduced to one-11th of the latter. This means that the powerrequirement for the multiple-gap cavity, allowing the largest powerloss, can be substantially equal to that for a single-gap cavity,moreover, with cavity resonators of such structure, an advantage isobtained that for some applications the operable frequency bandwidth canbe increased approximately 11 times owing to the low Q of the cavity.

Reference will next be made to FIGURES 3a and 3b, which illustrateanother embodiment of the present invention and in which referencenumerals 1, 2, 3, 4 and 6 indicate parts corresponding to those of theembodiment shown in FIGURES 2a and 2b carrying the same referencenumerals. In this second embodiment, each set of alternate aperturedmembers are interconnected by two supporting members, as illustrated.Such structure is not only desirable from the standpoint of mechanicalstrength but is a so effective to reduce the inductance of the circuitincluding the effective members compared to the previously describedstructure, in which alternate apertured members are interconnected by asingle supporting conductor.

The present invention may also be embodied in various modified forms.For example, in a modification shown in FIG. 4, in which referencenumerals 1, 2, 3, 4 and 6 indicate parts corresponding to those inFIGURES 2a and 2b carrying the same reference numerals, supportingconductors associated with any one of the apertured members have notnecessarily the same axial length nor are required to be symmetrical.The supporting conductors may have any appropriate configuration whileobtaining substantially the same effect as the previously describedcavity structures as long as the apertured members arranged alternatelyare electrically interconnected. Also it will be apparent to thoseskilled in the art that in cases where the apertures for passage of anelectron beam are relatively large in size, appropriate reticulate gridmeans may be employed to maintain the entire aperture area in anequipotential surface.

It will be appreciated from the foregoing description that the cavityresonator according to th present invention has various advantageousfeatures including compactness in size, relatively limited power loss, awide operational frequency range, lack of unwanted modes is the vicinityof the basic resonant frequency and high gain.

FIG. 8 illustrates a velocity modulation tube employing a number ofmultiple-gap cavity resonators according to the present invention. Inthis application, an input cavity resonator 9, a bunching cavityresonator 10 and an output cavity resonator Ii are arranged in axialalignment in the order named between an electron gun 7 for emitting anelectron beam and a collector 8 for collecting the latter after itspassage through the resonators. Each of the cavity resonators isconstructed ac cording to the present invention and includes a frequencycontrol mechanism 12 in the form of a plate which is disposed in thecavity in opposing relation to the working gaps. The frequency controlplate 12 provides an additional capacitance to the gaps and isadjustable to vary the resonant frequency of the resonator. Anelectromagnetic wave to be amplified is introduced through the coaxialinput terminal 13 formed on the input cavity 9 to excite the cavityresonators, applying to the respective gaps exciting voltagesalternately opposite in phase and substantially equal to each other. Thespace between any two adjacent gaps is so established as to give anelectron transit angle of approximately 1r radians. In this instance,since, as described hereinbcfore, the gap coefiicient is higher and theQ of the cavity is lower than those obtainable with a single-gap cavity,velocity modulation can be elfected upon the electron beam with higherefficiency over a frequency range wider than that of a single-gapcavity. The electron hcam is hunched for density modulation beforeentering the following cavity and,

through excitation of the latter or bunching cavity 10, is subjected toan additional velocity modulation. Thus, in the output cavity 11, thebeam contains all the density modulation components caused in therespective preceding cavities. The energy of the electron beam thenleaves the coaxial output terminal 14 formed on the output cavity 11 tobe supplied to the external load as a highfrequency exciting power. Inthe case where the elecron transit angle from gap to gap in each civityis slightly la ger than 1r radians, the Q of the cavity is reduced tofurther extend the amplification bandwidth because of the positiveloading. For the output cavity, which itself is heavily loaded with awider bandwidth, a negative beam loading can be used as long as itcauses no self-excitation by selecting an electron transit angle smallerthan 1r radians thereby to further improve the output efficiency.

It will be apparent to those skilled in the art that the presentinvention is applicable not only to amplifier tubes but also has manyother applications including microwave oscillators, frequencymultipliers, acceleration or deceleration of charged particles and phasefocusing, with such advantageous features as compactness in size, highefficiency and wideband characteristics.

It is to be understood that the invention is not restricted to thedetails set forth but many changes and modifications may be made withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

What We claim is:

1. A cavity resonator comprising a hollow conductor casing including endwalls extending at right angles to the axis of said casing, a number ofapertured members arranged along the path of charged particlescoincident with the axis of said casing so as to define a plurality ofgaps therebetween and having aligned apertures for passage therethroughof the charged particles, and supporting conductors interconnectingalternate ones of said apertured members and connected at one end to therespective opposite end walls of said conductor casing in spacedparallel relation to each other to form in said gaps electric fieldsalternately opposite in phase.

2. A velocity modulation tube of the type including an electron gun forproducing a beam of electrons, a collector for collecting said beam ofelectrons, input, bunching and output cavities arranged between saidelectron gun and said collector in the order named, a mechanism forintroducing an electromagnetic wave into said input cavity, and anoutput circuit for taking out the electromagnetic Wave amplified, saidtube being characterized in that at least one of said cavity comprises ahollow condoctor casing including end walls extending at right angles tothe axis of said casing, a number of apertured members arranged alongthe path of charged particles coincident with the axis of said casing soas to define a plurality of gaps therebetween and having alignedapertures for passage therebetween of the charged particles, andsupporting conductors interconnecting alternate ones of said aperturedmembers and connected at one end to the respective opposite end Walls ofsaid conductor casing in spaced and parallel relation to each other toform in said gaps electric fields alternately opposite in phase.

No references cited.

ELI LIEBERMAN, Primary Examiner.

S. CHATMON, JR., Assistant Examiner.

